虚继承

前一节的 student 示例程序看起来似乎已经解决了问题，但它存在着一些隐患。首先，在 TeachingStudent 类的 introduce() 方法里，我们不得不明确地告诉编译器应该使用哪一个属性。这对于 classes 属性来说是应该的，因为教一门课和上一门课有着本质的区别，而作为常识，助教生教的课程和他学的课程不可能一样！

但是我们再深入考虑下，既然在 TeachingStudent 对象里可以继承两个不同的 classes 属性，那它是不是应该有两个不同的 name 属性呢？答案：是！事实上，TeachingStudent 还真可以有两个不同的名字，这肯定不是我们在设计这个类继承模型时所预期的：Example1.cpp

TeachingStudent 类继承自 Teacher 和 Student 两个类，因而继承了两组 Person 类的属性，这在某些时候完全有道理，例如 classes 属性。但它也有可能引起麻烦，例如发生在 name 属性身上的情况。

#include <iostream>
#include <string>

class Person
{
public:
    Person(std::string theName);

    void introduce();

protected:
    std::string name;
};

class Teacher : public Person
{
public:
    Teacher(std::string theName, std::string theClass);

    void teach();
    void introduce();

protected:
    std::string classes;
};

class Student : public Person
{
public:
    Student(std::string theName, std::string theClass);

    void attendClass();
    void introduce();

protected:
    std::string classes;
};

class TeachingStudent : public Student, public Teacher
{
public:
    TeachingStudent(std::string theName1, std::string theName2, std::string classTeaching, std::string classAttending);

    void introduce();
};

Person::Person(std::string theName)
{
    name = theName;
}

void Person::introduce()
{
    std::cout << "大家好，我是" << name << "。

";
}

Teacher::Teacher(std::string theName, std::string theClass) : Person(theName)
{
    classes = theClass;
}

void Teacher::teach()
{
    std::cout << name << "教" << classes << "。

";
}

void Teacher::introduce()
{
    std::cout << "大家好，我是" << name << ", 我教" << classes << "。

";
}

Student::Student(std::string theName, std::string theClass) : Person(theName)
{
    classes = theClass;
}

void Student::attendClass()
{
    std::cout << name << "加入" << classes << "学习。

";
}

void Student::introduce()
{
    std::cout << "大家好，我是" << name << ", 我在" << classes << "学习。

";
}

TeachingStudent::TeachingStudent(std::string theName1,
                                 std::string theName2,
                                 std::string classTeaching,
                                 std::string classAttending)
                                 : Teacher(theName1, classTeaching), Student(theName2, classAttending)
{
}

void TeachingStudent::introduce()
{
    std::cout << "大家好，我是" << Student::name << "。我教" << Teacher::classes << ", ";
    std::cout << "同时我在" << Student::classes << "学习。

";
}

int main()
{
    Teacher teacher("小甲鱼", "C++入门班");
    Student student("迷途羔羊", "C++入门班");
    TeachingStudent teachingStudent("丁丁", "丹丹", "C++入门班", "C++进阶班");//两个名字出问题了 

    teacher.introduce();
    teacher.teach();
    student.introduce();
    student.attendClass();
    teachingStudent.introduce();
    teachingStudent.teach();
    teachingStudent.attendClass();

    return 0;
}

　　结果：

大家好，我是小甲鱼, 我教C++入门班。

小甲鱼教C++入门班。

大家好，我是迷途羔羊, 我在C++入门班学习。

迷途羔羊加入C++入门班学习。

大家好，我是丹丹。我教C++入门班, 同时我在C++进阶班学习。

丁丁教C++入门班。

丹丹加入C++进阶班学习。

请按任意键继续. . .

C++ 发明者也想到了这部分的冲突，因此为此提供了一个功能可以解决这个问题：虚继承（virtual inheritance）通过虚继承某个基类，就是在告诉编译器：从当前这个类再派生出来的子类只能拥有那个基类的一个实例。虚继承的语法：

class Teacher : virtual public Person
{ … }

这样做我们的问题就解决了：让 Student 和 Teacher 类都虚继承自 Person 类，编译器将确保从 Student 和 Teacher 类再派生出来的子类只能拥有一份 Person 类的属性！

栗子修改：Example2.cpp

#include <iostream>
#include <string>

class Person
{
public:
    Person(std::string theName);

    void introduce();

protected:
    std::string name;
};

class Teacher : virtual public Person//虚继承 
{
public:
    Teacher(std::string theName, std::string theClass);

    void teach();
    void introduce();

protected:
    std::string classes;
};

class Student : virtual public Person//虚继承
{
public:
    Student(std::string theName, std::string theClass);

    void attendClass();
    void introduce();

protected:
    std::string classes;
};

class TeachingStudent : public Student, public Teacher
{
public:
    TeachingStudent(std::string theName, std::string classTeaching, std::string classAttending);

    void introduce();
};

Person::Person(std::string theName)
{
    name = theName;
}

void Person::introduce()
{
    std::cout << "大家好，我是" << name << "。

";
}

Teacher::Teacher(std::string theName, std::string theClass) : Person(theName)
{
    classes = theClass;
}

void Teacher::teach()
{
    std::cout << name << "教" << classes << "。

";
}

void Teacher::introduce()
{
    std::cout << "大家好，我是" << name << ", 我教" << classes << "。

";
}

Student::Student(std::string theName, std::string theClass) : Person(theName)
{
    classes = theClass;
}

void Student::attendClass()
{
    std::cout << name << "加入" << classes << "学习。

";
}

void Student::introduce()
{
    std::cout << "大家好，我是" << name << ", 我在" << classes << "学习。

";
}

TeachingStudent::TeachingStudent(std::string theName,
                                 std::string classTeaching,
                                 std::string classAttending)
                                 :
                                 Teacher(theName, classTeaching),
                                 Student(theName, classAttending),
                                 Person(theName)//继承基类名字 
{
}

void TeachingStudent::introduce()
{
    std::cout << "大家好，我是" << name << "。我教" << Teacher::classes << ", ";
    std::cout << "同时我在" << Student::classes << "学习。

";
}

int main()
{
    Teacher teacher("小甲鱼", "C++入门班");
    Student student("迷途羔羊", "C++入门班");
    TeachingStudent teachingStudent("丁丁", "C++入门班", "C++进阶班");

    teacher.introduce();
    teacher.teach();
    student.introduce();
    student.attendClass();
    teachingStudent.introduce();
    teachingStudent.teach();
    teachingStudent.attendClass();

    return 0;
}